The present invention is directed to a pump-less hydraulic brake system for automobiles having two, diagonally divided brake circuits which are independent of one another.
For maximum safety, an automobile should be able to brake in the shortest possible distance under various instantaneous road conditions and vehicle weights. In stopping, the directional control of the vehicle is conditioned on the lateral guidance of the rear wheels. Maneuverability of the vehicle is conditioned on good lateral guidance provided by the front wheels, and should not be impaired. However, of the two directional stability is normally accorded the greater importance, and to attain such stability, a defined brake power distribution among the vehicle wheels is required under the particular weight conditions and for all types of braking of the vehicle.
The defined brake power distribution can be achieved with an automatic anti-lock system through the direct adjustment of the brake power distribution to the admissible slip between tire and roadway, i.e., adjustment to prevent locking of any of the vehicle wheels on braking.
The apparatus required and expense involved in providing an ideal automatic anti-lock system is considerable, and such a system has heretofore been used only in high speed and technologically sophisticated vehicles. As a result, attempts have been made to simplify the ideal automatic-anti-lock system, for example, by employing a diagonal control system or a rear axle control system. However, even the simplified anti-lock systems have not realized any considerable cost savings, as compared with the ideal systems, and at the same time they suffer from considerable functional shortcomings.
In an automatic anti-lock system with a diagonal control system, wheel sensors are provided in one of the two diagonal brake circuits and the brake pressure is controlled only in that one circuit. In such a manner, locking of the vehicle wheels associated with the controlled fluid line is prevented such that at all times a diagonally opposite front and rear wheel will not lock. This economy-type system offers improved operation under certain conditions, for example, when driving in a straight line, as compared with no system at all. However, on curved roadways, the direction of the curve determines the behavior of a vehicle thus equipped, in that only one of the diagonal brake lines has the anti-lock system present. Depending upon whether the car is braked in a right-hand curve or in a left-hand curve, the outside front wheel may lock if it is the one in the non-controlled brake circuit. If upon traversing a curve the outside front wheel happens to be in the non-governed brake line and locks, the maneuverability of the vehicle is considerably hampered. In the same system, on travelling through an oppositely directed curve, the outside rear wheel would be in the non-controlled brake line and thus can lock. If the outside rear wheel locks, the stability of the vehicle would be considerably impaired.
Due to the above described shortcomings, at the present time automatic anti-lock systems of any construction are currently employed only in comparative rare cases. Conventional brake systems currently utilized in most vehicles thus normally employ only a fixed brake power distribution among all the wheels, or employ a load-dependent rear brake power control, in the form of brake pressure reducers or brake pressure limiters, in order to prevent the rear wheels from locking before the front wheels. There is a considerably technological gap between brake systems employing an ideal automatic anti-lock arrangement and conventional brake systems with load-dependent brake power regulators or fixed brake power distribution.